1. Field of the Invention
This invention relates to an image pickup apparatus having an image pickup device having a photoelectric conversion element stacked above a semiconductor substrate and a charge storage section formed in the semiconductor substrate for storing a charge generated in the photoelectric conversion element for outputting a signal responsive to the charge stored in the charge storage section to the outside.
2. Description of the Related Art
In a single-plate color solid-state image pickup device represented by an image sensor of CCD type or CMOS type, three or four types of color filters are placed on a light reception section array for executing photoelectric conversion as a tessellated pattern. Accordingly, a color signal corresponding to the color filter is output from each light reception section and the color signals are processed, whereby a color image is generated.
However, with the color solid-state image pickup device with the color filters arranged as a tessellated pattern, if the color filters are primary color filters, about two thirds of incidence light are absorbed in the color filters and thus the light use efficiency is poor and the sensitivity is low; this is a problem. Since a color signal only of one color can be obtained in each light reception section, the resolution is also poor and particularly a false color is conspicuous; this is also a problem.
Then, overcome the problems, a stacked solid-state image pickup device of a structure wherein three photoelectric conversion layers are stacked on a semiconductor substrate has been researched and developed (for example, JP-T-2002-502120 and JP-A-2002-83946). This stacked solid-state image pickup device has a light reception section structure wherein photoelectric conversion layers for generating charges (electrons, holes) for light of B, G, and R are stacked in order from a light incidence face and moreover is provided with a charge storage section for storing a signal charge generated in each photoelectric conversion layer and a signal read circuit capable of reading a signal responsive to the charge stored in the charge storage section for each light reception section. Each photoelectric conversion layer is sandwiched between paired electrodes and one of the paired electrodes and the charge storage section are electrically connected, whereby the charge generated in the photoelectric conversion layer and moved to the one electrode is stored in the charge storage section.
With the described image pickup device, incidence light is almost converted into electricity for read and the use efficiency of visible light is near to 100% and moreover color signals of three colors of R, G, and B are provided in the light reception sections, so that a good image with high sensitivity and high resolution (false color is inconspicuous) can be generated.
The signal read circuit of the stacked image pickup device can be a CCD circuit for reading a charge stored in a charge storage section to a charge transfer section for transfer and converting the post-transferred charge into a signal voltage with an FD amplifier, etc., for output, a MOS circuit for converting a charge stored in a charge storage section into a signal voltage with a MOS transistor, or the like. To adopt the CCD circuit or the MOS circuit, after the signal read, it is necessary to sweep away the charge remaining in the charge storage section (transfer remaining charge if the CCD circuit is adopted) to a substrate, etc., and as an art concerning sweeping away of the charge, the art used with the single-plate image pickup device can be adopted.
For example, for the MOS circuit, a structure can be adopted wherein after an exposure time period expires, a signal responsive to the charge stored in the charge storage section in the exposure time period is output from the MOS circuit and then a reset pulse is supplied to a gate of a reset transistor for discharging an unnecessary charge to a reset drain.
FIG. 8 is a timing chart to describe the reset operation of the single-plate image pickup device. In FIG. 8, the case where the exposure time period and non-exposure time period of the image pickup device are controlled with a mechanical shutter is taken as an example.
When incidence light is incident on the image pickup device and the charge generated in the photoelectric conversion layer is stored in the charge storage section, the potential of the charge storage section rises as shown in the figure. After exposure terminates, a signal responsive to the charge stored in the charge storage section is output from the MOS circuit and the charge in the charge storage section is reset. When no light is incident after the reset operation, if a carrier generated in the photoelectric conversion layer is stored in the charge storage section in an instant, the potential of the charge storage section should not rise after the reset operation. However, a considerable time may be required until the carrier generated in the photoelectric conversion layer arrives at an electrode connected to the charge storage section and thus a carrier not moved to the charge storage section although it was generated in the photoelectric conversion layer during the exposure time period is stored in the charge storage section after reset as a residual image component. The residual image component mixes with a carrier stored in the charge storage section in the next exposure time period, so that the image quality is degraded. If the residual image component exists in the charge storage section, the potential of the electrode connected to the charge storage section rises and thus the bias voltage applied to the photoelectric conversion layer relatively lowers and a problem of lowering the carrier transport performance and making poor sensitivity can also occur. It is acknowledged that the residual image component in the photoelectric conversion layer decreases exponentially with the passage of time.
Then, it is considered that it is effective to prevent image quality degradation and sensitivity lowering by supplying a reset pulse once more when the residual image component in the photoelectric conversion layer sufficiently decreases. For example, as shown in FIG. 9, after a signal is read, when the residual image component in the photoelectric conversion layer sufficiently decreases, the reset operation may be performed once more and the next exposure may be started in a state in which almost no residual image component exists.
However, in so doing, the potential of the charge storage section fluctuates between the reset timings shown in FIG. 9 and thus meanwhile the force pulling out the residual image component in the photoelectric conversion layer to the electrode weakens. Consequently, the time until the residual image component in the photoelectric conversion layer sufficiently decreases is prolonged and the exposure time is shortened.